Information processing was monitored by averaged evoked response techniques The electroencephalographic (EEG) activity was recorded from left and right brain regions during memory and perception in normal subjects, patients with unilateral temporal lobectomy, and patients with neuropsychiatric disorders. EEG disturbances in brain-behavior relations in psychiatric patients were also evaluated, relating left and right brain dysfunctioning to activity to maladaptive ideative and emotional reactions, respectively. In temporal lobectomy patients, P300 amplitude was found to be inversely proportional to stimulus probability in the same way as for normal controls, and P300 amplitude was directly related to task difficulty. A double dissociation was found in the patients' responses to auditory and visual material. That is, for auditory material, left temporal patients showed smaller P300s over frontal scalp compared to right temporal patients and normal individuals, whereas, for visual material, right temporal patients manifested smaller P300s over frontal scalp compared to left temporal patients and or normal individuals. However, there were no consistent hemispheric asymmetries which distinguished the left from the right temporal patients, or either group from normal subjects. These data discount the hypothesis that medial temporal structures, including the hippocampus, serve as a sole generator of P300. More specifically the data indicate that processing of auditory and visual stimuli is dependent to some extent on the character of the material and the integrity of left and right brain mechanisms.